According to molecular orbital theory, what is the effect on a molecule when an electron is promoted from a bonding orbital to an antibonding orbital?

• A. The bond order increases, strengthening the bond.
• B. The molecule becomes more stable.
• C. The molecule gains a positive charge.
• D. The bond order decreases, weakening the bond.

Answer: (D)

Verified step by step guidance

1. Recall that in molecular orbital (MO) theory, electrons occupy molecular orbitals that are classified as bonding, antibonding, or nonbonding orbitals.
2. Understand that bonding orbitals stabilize the molecule by increasing electron density between nuclei, while antibonding orbitals destabilize the molecule by decreasing electron density between nuclei.
3. Recognize that bond order is calculated using the formula: \(\text{Bond order} = \frac{(\text{number of electrons in bonding orbitals}) - (\text{number of electrons in antibonding orbitals})}{2}\).
4. When an electron is promoted from a bonding orbital to an antibonding orbital, the number of electrons in bonding orbitals decreases by one, and the number in antibonding orbitals increases by one.
5. This change reduces the bond order, which corresponds to a weaker bond and decreased molecular stability.